1. Field of the Invention
The invention relates to a method of producing fertilizers from sewage.
The sewage may be domestic, industrial or agricultural sewage.
2. Description of the Prior Art
Domestic sewage represents the main problem in water pollution and its proportion of the total environmental pollution is estimated to be 63%. The remaining 37% is divided between industry and agriculture. Industry at least is endeavouring, albeit with considerable technical difficulty and financial expenditure, to restrict the discharge of contaminants into watercourses. The bulk of domestic sewage with its high proportion of natural biological constituents, which include some of thhe most valuable nutrients for the plant world, is for the most part, however, wasted unutilized.
The conventional treatment of sewage comprises introducing it into a clarifying tank, which brings about the settling of the solid particles, and then subjecting it to a "digestion" process in a digestion tank. The oxidation process thus aimed at here, which is necessitated by the presence of large quantities of what are known as "convenience chemicals of the affluent society" in domestic sewage, is considerably impeded or even rendered impossible. These "convenience chemicals", by which there are meant inter alia detergents, wetting agents, acids, alkalis, aromatics, and organic fats, etc., largely exclude even minimal oxidation processes. In addition it happens that digestion tanks constitute a breeding ground for bacteria and parasites. The result of such a digestion process is therefore not an activated sludge which is very suitable for further use but, rather, a biologically "dead" mass which, for example, is absolutely diametrically opposed to the requirements for natural plant growth. In this connection it has even been proposed to completely eliminate this non-utilizable mass by burning it, which, on the one hand, involves high costs (about 12 to 16 DM/m.sup.3) and, on the other hand, excludes any possibility of biological utilization.